The present invention relates to mechanical expansion anchors for support of roof bolts in pre-drilled holes in a mine roof, or the like, for 5 reinforcing and stabilizing the surrounding rock formation. More specifically, the invention relates to that class of mechanical expansion anchors having a shell assembly including two pairs of leaf members, the members of each pair being integrally connected to one another, and the two pairs being physically connected and held in assembled relation by a U-shaped bail having terminal ends fixedly attached to the leaf members adjacent their lower ends.
Mechanical expansion anchors are among the most common means presently used to support and reinforce underground rock structures such as mine roofs. Such anchors conventionally include a shell structure which is radially expanded into tightly gripping engagement with the wall of a hole drilled in the rock structure by movement of a tapered plug down the threads of a bolt as the latter is rotated, into the space surrounded by expansion leaves of the shell structure. In some shells all leaf members are integrally joined by a ring-like base member, while in others two or more separate leaf portions are physically attached and held in assembled relation by a bail member which extends over the plug and holds it in proper relation to the shell.
One type of prior art shell assembly includes two pairs of leaf members, the members of each pair being integrally connected to one another adjacent their lower ends by bridge portions which maintain the joined leaf members in laterally spaced relation with an open slot between opposing side edges. The two pairs of leaf members are joined to one another by a bail member having legs extending through the open slot between the leaf members of each pair and fixedly attached at their terminal ends to the bridge portions joining the two leaf members of each pair. The medial portion of the generally U-shaped bail passes over the top of the tapered plug and holds it in place with its smaller end extending into the space surrounded by the upper ends of the leaf members.
In prior art expansion anchors of this type, the expansion capability is limited to radially outward movement of the two pairs of leaf members in unison. That is, while each integrally attached pair of leaf members is moved radially outward into gripping engagement with the drill hole wall, each of the four leaf members of the two pairs cannot move independently, in four quadrants, to engage the wall. In order to achieve maximum gripping force from the external surfaces of the leaf members, it is desireable that each of the four leaf members move radially with respect to all of the other members, rather than only in cooperatively moveable pairs.
Other deficiencies have also been excountered in the use of bail-type expansion anchors having two pairs of interconnected leaf members. For example, it has been necessary in prior art designs to provide interlock means between the two otherwise independent pairs of leaf members to maintain side-by-side positioning when the unit is put into expansion. However, the interlocking portions of the two shell halves may tend to bind together, particularly in tight bore holes, when the two leaf segments are not in exact alignment or do not expand evenly. In such cases, a lockup may occur as one pair of leaf members is expanded outwardly while a downward force is exerted by the camming plug on the other pair.
Another problem which may occur is due to interference between the outer surface of the camming plug and the inner surface of the bridge portion connecting each pair of leaf members. When the plug is moved downwardly to the point that its lower end is adjacent the bridge portions connecting the lower ends of each pair of leaf members, an interference condition may arise, causing a galling restriction which is not acceptable for proper function of the unit.
It is a principal object of the present invention to provide a novel and improved mechanical expansion anchor of the type having two pairs of leaf members wherein the members of each pair are integrally connected to one another by bridge portions adjacent their lower ends, and the two pairs are physically attached by a bail member having two legs fixedly attached at their terminal ends to outer surfaces of the bridge portions.
Another object is to provide an expansion anchor of the foregoing type wherein the tapered camming plug which moves downwardly to effect shell expansion does not physically interfere with internal surfaces of the leaf members as the latter are caused to fully expand.
A further object is to provide a mechanical expansion shell comprising two pairs of leaf members wherein proper operation is ensured without the necessity of interlock means for maintaining alignment between the two otherwise independent shell halves.
Still another object is to provide a radially expansible anchor assembly wherein a cooperative expansion shell and tapered camming plug are so configured on their respective inner and outer surfaces that the likelihood of malfunction is minimized.
Other objects will in part be obvious and will part appear hereinafter.